Amnesiacs Remember

New Discovery Helps Amnesiacs Keep Their Memories
Newswise — Amnesia sufferers can lose their short-term memories within one minute of the initial thought. While such things as head injuries and strokes are known to cause amnesia, very little is known about why such severe memory loss occurs. However, a University of Missouri-Columbia researcher has discovered that some amnesiacs can maintain their memories for a longer period of time if they spend time sitting in a dark, quiet room.

I bet the reason this happens is because the hypothalamus is given more time to process the information.
In a light, activity filled room, an injured brain would be unable to 'buffer' since priority would be given to the visual information.
The darkened room would allow the hypothalamus to buffer the information long enough to get through the bottleneck.